The present invention relates generally to a bond coat composition for use with ceramic matrix composites. Specifically, the present invention relates to a bond coat composition for use with thermal barrier/wear coatings for ceramic matrix composites.
In some applications, a structural part is exposed to high surface temperatures on a heated surface of the part. These structural part constructions, such as ceramic matrix composites (CMCs), provide structural support for components associated with engine exhaust, and possibly the engine itself. An oppositely disposed surface of the part is cooled with a flow of cooling air. The maximum temperature reached by the part is determined by a balance between the amount of heat that enters the part from the heated surface, and the amount of heat removed by the cooling air flow over the cooled surface. Examples of such applications include combustors, nozzles, liners, exhaust flaps, seals and turbines in aircraft gas turbine engines.
It is known to apply a thermal barrier coating (TBC) to the heated surface of the part to serve as an insulation that reduces the heat flow into the part and allows it to operate in a hotter external environment. The TBC typically includes a metallic bond coat overlying the metallic substrate that forms the part, and a ceramic layer overlying the bond coat. The bond coat improves the adherence of the ceramic layer to the substrate. The ceramic layer, such as a zirconium-based ceramic, reduces the heat flow into the substrate from the hot surface.
Coated articles having a ceramic substrate, an intermediate thermal barrier coating overlying the substrate, and a low-emissivity metallic top coat over the thermal barrier coating are known. The available thermal barrier coatings typically utilize a ceramic layer overlying an intermediate metallic bond coat. The ceramic layer insulates the substrate, and the metallic bond coat improves the adherence of the ceramic layer to the substrate. The low-emissivity top coat reflects some of the radiant thermal radiant energy incident upon the coated article, so that the ceramic part below is exposed to less heat input. While operable, the available systems with metallic top coats have not been practical for use in high-temperature environments such as gas turbines, because the reflective metal degrades and/or volatilizes after brief exposure, frequently in minutes, to the high-temperature, corrosive environment.
It has been found that the bond coat is critical to the service life of the thermal barrier coating system in which it is employed, and is therefore also critical to the service life of the component protected by the coating system. The oxide scale formed by a diffusion aluminide bond coat is adherent and continuous, and therefore not only protects the bond coat and its underlying substrate by serving as an oxidation barrier, but also chemically bonds the ceramic layer. Nonetheless, aluminide bond coats inherently continue to oxidize over time at elevated temperatures, which gradually depletes aluminum from the bond coat and increases the thickness of the oxide scale. Eventually, the scale reaches a critical thickness that leads to spallation of the ceramic layer at the interface between the bond coat and the aluminum oxide scale. Once spallation has occurred, the component will deteriorate rapidly, and therefore must be refurbished or scrapped at considerable cost. Therefore, what is needed is a bond coat composition for use with CMCs that is inexpensive to make, easy to form and apply in a consistent manner, and tightly adheres both to the CMC substrate and TBC for providing enhanced durability improvement to the CMC.